Adding my own datapoint: I’ve absolutely had this happen on iPhone to iPhone where the message appeared to be sent on one device but the other received nothing
If I was being asked to debug this for someone I know, my first two questions would be “did your phone say delivered?” and “does the recipient have a Mac/iPad/etc that could’ve received the message?”; do you know the answer to either of these?
I worry that sounds accusatory but that’s not my intention, I’m curious and too tired to attempt to reword it.
